Propping for fuel economy

"As the watersports season end

"As the watersports season ends, I want to re-prop my boat for some long trips...for the purposes of saving fuel.

What I have now:
1996 Sea ray 175, 4.3L Mercruiser
14.25" x 19 pitch 3 blade aluminum prop
4900-5000 RPM WOT, moderately loaded boat

Basically, all I want to do is to cruise at 20-25 mph, with the maximum fuel economy possible. What prop would you guys recommend (I probably want to stick with low-cost 3 blade aluminum)?

(and, what would be a guess on how much better the fuel economy would be?)"

"fuel economy is not really ba

"fuel economy is not really based on prop pitch. It is based on how the boat is used.

There is what I would call a sweet spot where the motor runs at its most efficient state. That is a certain rpm. What Rpm That is the question.
Boat gas usage is measured in gallons per hour not in miles per gallon so the slower you go the less you use. Also in order to use less fuel you need to be on plane so less of the boat is in the water, the less boat the less drag. There is a balance of this and I am not sure you can gain anything noticable with a simple pitch change.

I do not believe there is a good answer to this question. To many variables.




If I were to guess I would say the best economy would be realised with the boat parked at a dock and sitting on the patio drinking while watching others cruise around the lake.

Just my opinion."

"I understand that boat gas us

"I understand that boat gas usage is usually given in gal/hour, but if I have a 100 mile trip to make, the gal/mile number (inverse of MPG) is what matters there.

If I run at 25 mph at 3500 RPM with my current prop, and (for the sake of argument) I run 25 mph at 3000 RPM with a higher-pitched prop, won't I be consuming less fuel in the second case?"

Not neccaserily (spelling&

Not neccaserily (spelling).

The motor may actually use less fuel at 3500 rpms because of less boat in water less drag and maybe that is the cams sweet spot for optimal performance. To many factors and I also would not think and I know it is hypathetical that a 500 rpm change would do much for fuel economy.

Hull design and power is mostly what makes the big differences and even small ones.

To make less of the boat in the water at a lower rpm would make the "mpg's" improve. Not just the rpms. It is very complicated with boats not quite as simple as with auto's.

Wish it was becuase I would like nothing more than to stop using 15-20 gallons when cruising at 1500 rpms for 5-7 hours by changing the prop pitch.......... I would be able to buy more beer!!!!

"

I agree within reason,

"

I agree within reason, of course...at 20-25 mph, my boat is on plane, so I had removed that from the equation; given a constant speed and the same water conditions, the amount of power output by the engine should be the same, and the amount of boat in the water should be the same...the only thing different should be the RPM's turned because you are getting more bite with each turn on the prop. The same as running 20 mph in 2nd gear vs. 20 mph in 3rd right?

[not trying to be difficult I promise!!] "

"A prop change alone is not go

"A prop change alone is not going to change the power needed to move your boat through the water at a given speed.

You may find that you are turning fewer RPMs with a bigger prop, but the throttle position (the one thing that directly affects how much gas you are pumping into the engine) isn't going to change much, just by changing the prop.

A slightly higher pitch may help a little, if you happen find a sweet spot where the engine runs the most efficient, but by the time you're ready to put the other prop on in the spring time, you'll probably have not saved enough gas make the purchase of the new prop cost effective.

You'd get more bang for your buck in trim tabs and a hydro foil if you don't already have them. They will also help pulling skiers, so you'll see continuing returns on the investment.
With both, or either, you will find your are using less throttle to achieve the same speed... therefore less fuel.
Trim tabs make the boat effectively longer, without adding surface area to the hull. (Same amount of drag with a more effectient shape.)
Hydrofoils help lift the stern out of the water, and keep a slightly higher pressure on the front side of the prop, reducing cavitation.

If you do opt to go for the bigger/steeper prop, don't make a huge jump. You'll find yourself using more fuel, and the boat will be a complete slug... Like, WOT at 3500 rpm, struggling to stay on plane."

"Troy

I agree with the trim

"Troy

I agree with the trim tabs but I totally diagree with the hydrafoil. This divice causes high speed steering issues and is NOT recommend by MERCURY or me for that matter!
IF and that is a big IF they are to be used it is usaully on an under powered boat as a cheat to get it up on plane quicker/easier.


I have read the bulliten! And I have driven enough boats with them installed to know what they are talking about.

The trim tabs might help BUT like you said, Just how much will be gained???????"

"Based on what you guys are sa

"Based on what you guys are saying, it looks like it won't make much of a difference, so I'll keep the prop I have.

I have a hydrofoil (SE Sport); don't have trim tabs. The main reason I got a hydrofoil is that I spend a lot of time in the 10-12 mph range, pulling small children on tubes. At those speeds, it's hard to keep the boat up on plane, and I believe the foil helps me there."

It is your boat and you have a

It is your boat and you have a right to do what YOU want to it to help your situation.

If it works for you and you understand the potential risks than enjoy it.

Just like me I know drinking and driving do not mix BUT I choose to drink a few "beverages" while burning a ton of gas just to get a sun tan........... A complete oxymoron??

"high-speed steering issue

"high-speed steering issues"...

Got a copy of that bulletin?

It was released in the early 9

It was released in the early 90's when I was doing this for "a living" professionally.

If you have doughts take one with and without for a ride and see/feel the diference. Especially boats with NO power steering!!!!

"OH and one more thought,

I

"OH and one more thought,

If it was such a great thing to have on a boat dont you think the oem's would just make the cavatation plate that big to begin with???

Reason they dont I bet is because of high speed steering concerns."

"Relax, I just want to read it

"Relax, I just want to read it.

When I bought my boat, it didn't have a hydrofoil. After the first couple of seasons, I installed one. I think that meets your with and without criteria. Personally, I consider "High-speed" and "Steering" terms not belonging in the same sentence, so I've never really tested the theory. I'd like to learn more before I do so the hard way.

What's a "dought"?"

It is after lunch time garble.

It is after lunch time garble......can you say learn how to spell?

"doubt"

Been along time since I learned how to spell Used the old method of sounding it out???

"I used a hydrofoil on a 470 p

"I used a hydrofoil on a 470 powered (read underpowered) boat of 20 feet and it made a remarkable difference in the amount of boat in the water on plane. Far better cruise. Made no difference whatso ever on a bigger boat i also owned.

No doubt the OEMs would have done it from the factory if it was the end all.

Regarding mileage... sack up, gas up and make that run!"

"Daniel, I have an Bayliner 18

"Daniel, I have an Bayliner 185 with a 4.3 mercruiser and a hydrofoil. I went from stock 14.x23 alum prop to a 14.25 x23 stainless . That change dropped me 250 rpm topend and I decreased my fuel consumption running 30mph at 3000rpm. I did give up about 5mph top end and a little of the hole shot, but i'm burning less fuel and I can swap the prop back if I want to go fast"

"I'd still like to know wh

"I'd still like to know what the "High-speed steering" issue is.
- Does the boat get hard to turn, not turn at all, turn the wrong way?
- What speed(s) does this phenomenon occur?
- What hull size/shape is susceptable?
- Any other factors?"

"After installing a a hydrfoil

"After installing a a hydrfoil onto a cavatation plate on an outdrive the first thing you will notice is how fast the boat comes on plane.
Second as you increase speed you should notice that the boat wants to "nose dive" a bit due to the hydrafoil. ( I call em wale tails). In other words the boat will only lift so far out of the water due to the HF trying to push it down.

When you are at a high rate of speed and you make a turn the boat will start to turn and then it will continue to turn but in a somewhat uncontrolled manner, on its own if you will like it wants to just turn harder without you turning the wheel. You could very easily want to turn so far and stop but you will have to physically turn the wheel or hold it and have a difficult time bringing the steering back to center

This proplem is extremely exaggerated if there any slop in the gimbal ring to steering pin.

Again it shows this more with non power steering models."

"Sounds more like a lack of un

"Sounds more like a lack of understanding on the part of the operators.
Not arguing that the things you describe don't happen, but,
WRT the nose diving, a little trim adjustment, unless you like porpoising, fixes that, and gives the bow a better entry. Also, it shifts the weight of the boat farther aft, where it belongs, so more of the sides of the hull are in the water and thereby making it more stable (ie., controllable).

WRT the oversteering, two solutions: slow down to make a tight turn, or don't turn so sharply. More frequent, less drastic steering orders are almost always better than throwing her hard over and expecting miracles. If a situation requires drastic turning, to perhaps avoid a collision, pulling the throttle back is the first step in the maneuver. I think the hydrofoil in this case is merely amplifying the real underlying issue.

The only situation I can image where this would be a problem is if you're trying to pull a skier and there isn't enough sea room to maintain speed and make the turn safely. You're either in the wrong place to be doing what you're trying to do, or you're trying to use equipment not suitable for the task. If the installation of the hydrofoil keeps you from operating the boat the way you want to, then that solutions seems fairly simple. If you want to be economical at the same time, then you need to realize that in most single engine stern drive boats, the lever you operate with your right hand is directly connected to your wallet."

"your arguments sound good in

"your arguments sound good in a PERFECT world but the reality is people are people and they do exactly as I described.

On the subject of the bow bieng forced down due to the HF, if you really think about the way it all works you will see my point and it is not just a point it is many first hand experiences.

When I worked in the business back when dinosaurs roamed the earth, EVERY boat I worked on for what ever reason was lake tested. We never fixed A problem and just gave the boat back to the customer. We made sure there were no other issues.
Good business practice!! Anyway by way of lake testing any and all boats serviced I personally drove to many with the hydrafoil attached to the anti-cavitation plate and when at high speed the bow gets pushed down and the overall full speed is reduced in allmost all situations.

maybe on the bigger boats above 22 feet it may not be much of an issue but on boats smaller than 22 ft it definatly is. Especially in the 16 to 21 ft range."

"I think we're in agreemen

"I think we're in agreement, just arguing different perspectives.

I really would like to read the Merc Bulletin and get their take it. Anyone out there have it or access to it?"

"I think the hull design might

"I think the hull design might have as much to do with control impairment as the hydro foil. I've driven several boats over the years(maybe 4 or 5) including my own, at all kinds of speeds, and have never experienced what you described. Maybe I just haven't pushed far enough, or maybe the geometry of those boats was such that the problem was abated. My 18ft bow rider with a 4.3 came with a whale tail on it(i bought it used), and it handles like a sports car at speed. The fastest i've ever pulled turns is about 37(i top out at 50, maybe 51 if im lucky), and they were on the steep side-- steep enough to require more throttle to maintain speed. My boat has either a 12 or 14 degree dead rise(cant recall), and is a nice steep vee. Maybe i'm just too conservative of a driver to get into the speed range/turn combo to experience it. Certainly though when trimmed out, my bow never sinks. If im running around with the prop down, it will run low but that's just the nature of the beast(isn't it??). Anyways, i've never had a problem with my boat's handling at any speed.

On to the question, wouldn't the orig poster have to re-gear his drive AND change the prop to accomplish what he wants? Changing the ratio of the lower unit certainly would be the equivalent of 2nd vs 3rd gear in a car, no? I should think that would be a heck of cost for marginally better economy though

--chris"

"It does not quite work that w

"It does not quite work that way. You would think it does but it does not.

I can not explain the reasons as I am not a boat designer/engineer.

If you alter the ratio in the outdrive you still need to reach a certain speed to get the boat on plane. All the ratio will do is to lower or raise the rpms required but the fuel usage will be very close to the same. One way it will rev very high very qickly to achive the speed needed and the in the other direction it will take a longer time to reach the speed needed even at a lower rpm so more or equal fuel will be consumed. I think it has to do with the water friction/surface tension vs air.

On a car you can put very tall tires and very skinny tires, you can adjust the areodynamics of the body and so on. Also once you break the inertia of starting to roll in an auto then you are only dealing with the tire contact and wind resistance and wieght of vehicle. Remeber to achive max eficiency in a boat, as much of the boat as possible needs to be OUT of the water and to do this requires power and uses fuel.

Change the hull design for this purpose and play with prop/gear changes and then with the three you could really make some increases in fuel economy.

I will reiterate............Gallons per hour for boats, Mpg for auto's.

They do not use that terminology just because........

As far as your 18 BR with the tail, Have you ever tried it without it?

If not then you really do not have a reference point to go by.

I will say that there may be some designs that may not have as negetive affect as I had seen when I used to do this.

As far as a bulliten goes Mercury SPECIFICALLY said NOT to install these on any mercury product PERIOD. It will void any warrenty issues due to steering and or outdrive/gimbal ring premature failures.

They basically were saying if a marine business installs one then they take on all responsibilties as the "manufacturer" of the outdrive. The same type of bulliten came out for gimbal hounig extensions that put the outdrive back further by at least 3 to 6 inches.

Mercs choice because they have a lot of leagle issues regarding anything you can think of.

Example: A boat rental place a couple of miles from us rented a boat to a couple. They were using it and she repeatedly ran over her partener, Yes ran him over sevral times. She tried to blame it on the boats steeering.

We serviced the boat before and stored it after the "accident". Many limo's showed up one day and a dozen or so suits got out and a couple of techs/engineers to scour over the boat for anything that could have caused the problem.

No blame was assigned to the boat or anyone who came in contact with it before they rented it. You can figure the rest of the story for yourselves...............wild woman!!

So to avoid issues such as this they send out bullitens all the time about not installing aftermarket parts to there designs. its called CYA..... Cover Your A$$"

Wonder what in the world that

Wonder what in the world that guy did to elicit that kind of fury!!! Ouch

"I was trying real hard to sta

"I was trying real hard to stay out of this one, but you guys suckered me in.

I guess everyone has an opinion about hydrofoils, and here is mine, though no one solicited it.

I have had one installed on my last two boats including the present 20' Blue Water Mirage. Its an Attwood "V" style; not the flat type. It's installed in the normal spot on the aft end of the lower ventilation plate. The fins angle upwards at about 15 degrees from horizontal.

As noted above it sure helps get her up on plane fast, which is greatly appreciated by skiiers and boarders.

I have watched the outdrive over the stern of the boat a couple of times, and at plane, the hydrofoil is almost completely out of the water.
That's right; OUT OF THE WATER!

This means of course that it has absolutely no effect whatsoever as soon as you are up on plane. It does not affect speed, steering, plowing, or any other facet of the boat's handling.

Just thought I would mention this fact. I don't know if other makes function the same way.

Rod"

NO

NO